The present invention relates to a signal conditioner for a linear variable differential transformer with a center-tapped secondary or two or more secondaries and a relatively moveable core.
A linear variable differential transformer (LVDT) is an electro-mechanical transducer which allows for the measurement of very small motions in a structural mechanical device. LVDTs are used for measuring position, pressure, load weight, angular position, and acceleration. In an LVDT, mechanical motion of a core is translated to an electrical signal which contains position information in the phase and amplitude of an output AC waveform.
In order to obtain useful information from the LVDT, a signal conditioner is required. There are three basic operations which are performed by an LVDT signal conditioner. First, the conditioner must drive the primary winding of the LVDT with an AC signal. Second, the conditioner obtains position information from the LVDT by detecting the difference between the voltages developed across the two secondary windings. This difference is an AC signal which has a phase relative to the drive signal that indicates the LVDT core is above or below the center position. The amplitude of this AC signal indicates the distance of the LVDT core from the center position. Finally, the difference is then converted into a useful electrical signal that is proportional to the position of the moveable core within the LVDT.
At some point in the signal conditioning process, the AC signals obtained from the secondaries must be rectified and filtered to generate an output DC signal. In U.S. Pat. No. 3,079,545 to Kretsch et al., the signals from the secondaries are rectified first, and an output signal is found from the difference between these rectified signals obtained by subtracting one DC voltage from the other. However, such a design results in lower accuracy than more modern devices, since there are always differences in the DC voltage generated by seemingly identical rectifiers from a given AC voltage of a given amplitude. This would have the effect of reducing the accuracy of measurements that are based in the difference between two DC voltages when these voltages are derived from AC voltages of nearly equal amplitude. By directly sensing the difference between the secondary AC voltages, and only then employing a single rectifier to generate the necessary DC voltage, more modern devices eliminate this source of error.
In the more recent state of LVDT signal conditioner technology, the conditioner senses the difference in amplitude between the two secondary AC voltages. This difference is then rectified. To preserve the phase information contained in the AC voltage, the AC signal is rectified with the aid of a synchronous demodulator. Such an LVDT signal conditioner is shown in Signetics product NE/SE 5520 and FIG. 3. However, the use of a synchronous demodulator increases the circuit complexity and the power requirements.
Accordingly, it is an object of the present invention to provide an LVDT signal conditioner which has reduced circuit complexity and power requirements, while retaining a high accuracy of measurement.
Another object of the present invention is to provide a signal conditioner which uses a summing amplifier in such a way that it serves the dual purpose of preserving the phase information in the difference between two secondary AC voltages, while shifting the level of the DC output signal.
These and other objects are achieved in the present invention which provides a signal conditioner for a linear variable differential transformer having a difference circuit for sensing differences in amplitude between secondary alternating current voltages in the first and second secondaries and a phase comparator for comparing the phase of a difference signal from the differential amplifier with the phase of one of the secondary alternating current voltages to produce a position signal. A peak detector is connected to the output of the phase comparator.
In certain preferred embodiments, the difference circuit includes a first differential amplifier for forming a difference between the first and second secondary voltages multiplied by a first constant, and a second differential amplifier for adding the first secondary voltage multiplied by a second constant to the difference from the first differential amplifier. The addition of the first secondary voltage multiplied by the second constant shifts the level of the DC output voltage position signal so that the position signals are always the same polarity.
Other objects, features and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, an embodiment in accordance with the present invention.